Railcar couplers are disposed at each end of a railway car to enable joining one end of such railway car to an adjacently disposed end of another railway car. The engagable portions of each of these couplers are known in the railway art as knuckles. For example, railway freight car coupler knuckles are taught in U.S. Pat. Nos. 4,024,958; 4,206,849; 4,605,133; and 5,582,307.
Coupler knuckles are generally manufactured from a cast steel using a mold and three cores that produce the interior spaces of the knuckles. These three cores typically make up the rear core or “kidney” section, the middle core or “C-10” or “pivot pin” section, and the front core or “finger” section. During the casting process itself the interrelationship of the mold and three cores disposed within the mold is critical to producing a satisfactory railway freight car coupler knuckle.
The most common technique for producing these components is through sand casting. Sand casting offers a low cost, high production method for forming complex hollow shapes such as coupler bodies, knuckles, side frames and bolsters. In a typical sand casting operation, (1) a mold is formed by packing sand around a pattern, which generally includes the gating system; (2) The pattern is removed from the mold; (3) cores are placed into the mold, which is closed; (4) the mold is filled with hot liquid metal through the gating; (5) the metal is allowed to cool in the mold; (6) the solidified metal, referred to as raw casting is removed by breaking away the mold; (7) and the casting is finished and cleaned which may include the use of grinders, welders, heat treatment, and machining.
In a sand casting operation, the mold is created using sand as a base material, mixed with a binder to retain the shape. The mold is created in two halves—cope (top) and drag (bottom) which are separated along the parting line. The sand is packed around the pattern and retains the shape of the pattern after it is extracted from the mold. Draft angles are machined into the pattern to ensure the pattern releases from the mold during extraction. In some sand casting operations, a flask is used to support the sand during the molding process through the pouring process. Cores are inserted into the mold and the cope is placed on the drag to close the mold.
When casting a complex or hollow part, cores are used to define the hollow interior, or complex sections that cannot otherwise be created with the pattern. These cores are typically created by mixing sand and binder together and then filling a box shaped as the feature being created with the core. These core boxes are either manually packed or created using a core blower. The cores are removed from the box, and placed into the mold. The cores are located in the mold using core prints to guide the placement, and prevent the core from shifting while the metal is poured. Additionally, chaplets may be used to support or restrain the movement of cores, and fuse into the base metal during solidification.
The mold typically contains the gating system which provides a path for the molten metal, and controls the flow of metal into the cavity. This gating consists of a down sprue, which controls metal flow velocity, and connects to the runners. The runners are channels for metal to flow through the gates into the cavity. The gates can control flow rates into the cavity, and prevent turbulence of the liquid.
After the metal has been poured into the mold, the casting cools and shrinks as it approaches a solid state. As the metal shrinks, additional liquid metal must continue to feed the areas that contract, or voids will be present in the final part. In locations with heavy thick metal sections, risers are placed in the mold to provide a secondary reservoir of liquid metal. These risers are the last areas to solidify, and thereby allow the contents to remain in the liquid state longer than the cavity or the part being cast. As the contents of the cavity cool, the risers feed the areas of contraction, ensuring a solid final casting is produced. Risers that are open on the top of the cope mold can also act as vents for gases to escape during pouring and cooling.
In the various casting techniques, different sand binders are used to allow the sand to retain the pattern shape. These binders have a large effect on the final product, as they control the dimensional stability, surface finish, and casting detail achievable in each specific process. The two most typical sand casting methods include (1) green sand, consisting of silica sand, organic binders and water; and (2) no-bake or air set consisting of silica sand and fast curing chemical adhesives. Traditionally, coupler bodies and knuckles have been created using the green sand process, due to the lower cost associated with the molding materials. While this method has been effective at producing these components for many years, there are disadvantages to this process.
Many knuckles fail from internal and/or external inconsistencies in the metal through the knuckle. These inconsistencies can be caused when one or more cores move during the casting process, creating variances in the thickness of the knuckle walls. These variances can result in offset loading and increased failure risk during use of the knuckle.
Traditionally, each of the three cores needed to be set in a separate print in the mold which helps maintain each core's position. Furthermore, additional support mechanisms, such as manually inserted nails, are necessary to avoid shifting. These techniques are labor intensive and allow for human error.
Earlier designs may also allow turbulence in the flow of molten steel during the pour due to the sharp transitions in certain areas. When metal fills the molds under high velocity, it creates turbulence. Any sharp or abrupt transition in the molds or cores also creates turbulence, and/or pressure gradients that can also cause the cores to shift. Furthermore, the turbulence and pressure gradients can cause mold erosion, inclusions and reoxidation defects. These problems can cause solidification issues such as shrinkage and porosity, which in turn can lead to knuckle failure.
The issues above can all result in casting inconsistencies in the knuckle core surfaces. The ramifications of such inconsistencies and the low fatigue strength of the resulting parts can be extremely expensive, as The Association of American Railroads (AAR) has strict standards as to when a part must be scrapped and replaced. The 2011 Field Manual of the AAR notes at Rule 16, Section A, that “knuckles found broken or with cracks in any area . . . determined by visual inspection and/or by utilizing non-destructive testing as defined in AAR Specification M-220 shall be scrapped, (emphasis added). Due to these strict standards, and the expense of replacing these parts in the field, there is an ongoing need to improve the strength and/or fatigue life in coupler knuckles as well as a need to improve the design of the cores used to form the knuckles.